The present invention relates to a thermal insulating material based on diatomaceous earth, particularly a cellular thermal insulating material and method for its production.
In the production of thermal insulating materials, one usually begins with a framework or matrix former and a bonding agent which are expanded in order to obtain structures with air-filled pores that have particularly good heat insulating properties.
Production of a thermal insulating material of SiO.sub.2 -containing material (framework former), calcium hydroxide, water, foam and reactive aluminates contained in regulated set cement is known from DE-A1-43 39 137. The pourable crude mixture made from these materials is filled into forms. After sufficient hardening of the crude mixture, the blank pieces are removed from the forms and autoclaved. Slow heating (corresponding to the saturated heat curve) occurs for autoclaving, followed by a holding phase of 8 hours at about 14 bar (corresponding to about 198.degree. C.) and then a cooling phase of two hours. According to DE-A1-43 39 137, a quartz powder with a maximal particle size of about 20 .mu.m, particularly from 2 to 3 .mu.m, is used as a SiO.sub.2 -containing material. Calcium hydroxide is used in the crude mixture in an amount which is essentially stochiometric to the amount necessary for the complete conversion of the quartz powder to tobermorite and of the reactive aluminate contained in the regulated set cement to monophases and (secondary) ettringete. A practically complete material conversion of quartz and calcium hydroxide to tobermorite is achieved with steam curing in the autoclave.
DE-A1-44 08 088 relates to a method for the production of a porous mineral light insulating board which starts from a bonding agent slurry of cement, quartz powder (as a framework former), calcium hydroxide and water, mixes this with a separately produced foam and makes a formed cake for the light insulating boards from the obtained mixture. After stiffening of the formed cake, this is cut into individual light insulating boards which are then cured in an autoclave. DE-A1-44 08 088 also teaches a water-repellent and/or hardening impregnation as a final treatment for the autoclaved light insulating boards.
Thermal insulating material densities between 100 and 200 kg/m.sup.3 are achieved with the methods known in the art. For example, a density of 100 kg/m.sup.3 in the dry state is given in DE-A1-44 08 088 for a light insulating board. The light insulating boards produced in this manner typically have a specific thermal conductivity of 0.045 (W/m.sup..multidot. K).
However, such light insulating boards have several serious disadvantages. For one, the specific thermal conductivity is still comparatively high and not suitable at all for thermal insulation where large temperature gradients exist. Furthermore, as a result of the specific density of the quartz (ca. 2.7 kg/m.sup.3), insulating boards produced with quartz powder are breakable because only 36.7 l of solid matrix is available in an insulating board with a density of 100 kg/m.sup.3 with an air volume of 962.5 l in the cells of the boards. In order to achieve maximal strength of the matrix, the quartz sand must also have a particle size in the range from 2 to 8 .mu.m. On the one hand, this leads to problems (for example airborne dust) in handling the powdered quartz sand in the production method and, on the other hand, to dust formation in breakage or with abrasive wear of the insulating boards. Further, quartz dust (airborne dust) leads to serious lung diseases (silicosis) with long term exposure.
Additionally, conventional mineral insulating boards lead to insufficient adhesion between boards and the applied plaster which is often due to dust formation.
Methods which employ quartz powder as a framework and/or matrix material have the further disadvantage that the quartz itself does not have any hydraulic properties and, therefore, long pre-hardening times and/or large amounts of bonding agent are sometimes necessary.
The object of the present invention is to provide a thermal insulating material having improved thermal insulating properties, improved adhesion properties (with plaster) and an improved mechanical strength, especially at high temperatures, and whose use does not involve any health risks.
The object of providing a method for the production of such a thermal insulating material is also connected with this.